Prior to setting forth the background of the invention, it may be helpful to set forth definitions of certain terms that will be used hereinafter.
The term ‘Microstrip’ as used herein is defined as a type of electrical transmission line which can be fabricated using for example printed circuit board technology, and is used to convey microwave-frequency signals. It consists of a conducting strip separated from a ground plane by a dielectric layer known as the substrate.
The term ‘Stripline’ as used herein is defined as a transverse electromagnetic (TEM) transmission line medium. A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes. The insulating material of the substrate forms a dielectric substrate. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line.
Filtering radio frequency signals is a fundamental need in radio frequency and microwave systems. The most common filters in the microwave range are bandpass filters, which pass a specific range of frequencies, and attenuate the signals at lower or higher frequencies. Design of microwave filters is commonly implemented using “resonators” and coupling structures, which control the signal passage between the resonators. In most basic structures the resonators form a chain and only the adjacent resonators are coupled. In more advanced structures coupling between nonadjacent resonators is added, to introduce nulls in the rejection bands.
There are numerous techniques of implementing resonators, such as cavities and transmission line segments (e.g. ½ wavelength for open-open lines and ¼ wavelength for shorted-open lines). The resonators can be reduced in size by use of dielectric materials, such as using ceramic materials or printed circuit substrates. Transmission line structures can be further shortened by using stepped-impedance resonators, which emulate lumped L-C structures. Folding the resonators, such as in “hairpin” filters is possible. These typical resonator structures are exemplified in FIG. 1. In FIG. 1a a resonator is shown comprising a quarterwave transmission line 100 connected to ground at one end 101 and left open at the other end 102. FIG. 1b shows a shortened (less than quarterwave) resonator in which the open end 102 is further loaded by a capacitor to the ground. FIG. 1c shows a halfwave resonator in which the transmission line 100 is shorted to ground at both ends 101a, 101b, while FIG. 1d shows a halfwave resonator in which the transmission line 100 is open at both ends 102a, 102b. FIG. 1e shows a stepped impedance resonator, comprising a grounded high-impedance (inductive) section 100a and an open low impedance (capacitive) section 100b. FIG. 1f shows a stepped-impedance equivalent of a halfwave transmission line open at both ends. FIG. 1g illustrates a hairpin resonator, where folding is utilized do reduce the physical extent of the resonator, while FIG. 1h shows a further folded hairpin structure.
Let us briefly review the main transmission line structures applicable to printed circuit technology. The most popular structure is the “microstrip” structure, the cross section of which is illustrated in FIG. 2a, in accordance with the prior art. The signal line 200 is situated at the outermost (top or bottom) side of a printed circuit, with a dielectric substrate 202 separating between the signal line and the ground plane 201. Another popular structure, “stripline”, is shown in FIG. 2b. In this case the signal line 200 is surrounded on both of its sides by dielectric material 202 and groundplanes 201a, 201b. The stripline structure is inherently shielded by its groundplanes. FIG. 2c shows another popular structure, a coplanar waveguide (CPW). In this structure the signal line 200 and the ground conductors 204 are all situated on the same metal layer 205 of the printed circuit. In the CPW structure the fields are primarily confined to the gap between the signal line 200 and the groundplane conductors 204. Practically, the CPW structure is supported by the dielectric structure of the printed circuit. CPW structure has variants combining it with groundplanes. In FIG. 2d a CPW with a single groundplane 201 is shown, being a hybrid of a CPW and a microstrip. In FIG. 2e a CPW with two groundplanes 201 is shown, being a hybrid of a CPW and a stripline. Finally, FIG. 2f shows an example of a multilayer printed circuit board (PCB) incorporating multiple transmission line structures by stacking them in the vertical domain. The line in the uppermost layer is a grounded CPW, followed by a stripline, followed by a doubly-grounded CPW, ending with a microstrip at the bottom layer. Several observations are appropriate here. First, the groundplanes 201 shield between the different transmission lines, creating thus very high isolation between signals at the different layers. Another feature is the metallic sidewalls 203, which equalize the potential between the groundplanes and prevent the formation of signals between the groundplanes, which act as a parallel-plate TEM waveguide. The sidewalls 203 are typically implemented as multiple consecutive “vias” (metallic posts interconnecting layers in the PCBs).
Inter-resonator coupling structures are equally diverse. Cavity resonators are often coupled by slots in the inter-cavity walls. Transmission line resonators are often coupled by predominantly inductive (current based, such as proximity between ends shorted to ground), predominantly capacitive (voltage based, such as proximity between open ends), or distributed coupling (such as parallel ¼ wavelength sections). FIG. 3 exemplifies common cases according to the prior art of coupling between transmission line resonator structures. FIG. 3a shows the case of primarily inductive coupling between the shorted ends of quarterwave resonators. FIG. 3b exemplifies capacitive coupling along transmission lines near their open ends. FIG. 3c shows capacitive coupling between open ends of transmission lines. FIG. 3d illustrates primarily magnetic coupling between the high-impedance sections of stepped impedance resonators, while FIG. 3e shows a capacitive coupling between the low-impedance line sections of stepped impedance resonators.
Let us address the embodiment of coupled transmission lines in physical structures according to the prior art. FIG. 4 addresses various cases of printed circuit transmission lines. FIG. 4a shows side-coupled microstrip lines. FIGS. 4b and 4c show coupling between stripline transmission lines—FIG. 4b illustrates side coupled lines, used for low-to-medium coupling, while 4c illustrates broadside-coupled lines, typically used for medium-to-high coupling factors.
For narrowband low-loss filters air-filled resonators are common. For medium bandwidth, low-loss, high dielectric constant dielectric materials are used. For medium bandwidth and above printed circuit techniques are commonly used. Microstrip transmission lines are popular, and many microstrip PCB filter structures were developed. The resonators are commonly placed side by side for coupling. “Stripline” resonators, in which the transmission line is sandwiched between groundplanes, are also used. In this case, the lines are coupled by lateral proximity, or by partial overlap between resonators in adjacent layers (“broadside coupling”).
Multilayer printed circuit board technology is well developed and is suitable for mass production. Another well-developed multilayer circuit technology suitable for filter production is ceramic technology, such as LTCC (low temperature co-fired ceramics). The number of layers used relates directly to manufacturing cost.
Multilayer stripline circuits with multiple ground planes are well known, including uses for filter applications. An example of such structure is described in U.S. Pat. No. 7,755,457 (to Harris), where ground layers alternate with signal carrying layers. The signal carrying layers then contain the resonant structures needed to perform the filtering functions. The signals are conveyed from the outermost layer to inner layers using via pins.
In U.S. Pat. No. 5,719,539 (to Matsushita Electric Company) multilayer filter structures are described. In some embodiments of this patent, the layers are divided into ground layers, resonator layers and coupling capacitor layers. In some embodiments resonators are coupled through slots in groundplanes separating the resonators. In other embodiments stripline resonators in adjacent layers are provided, sharing common groundplanes which are coupled by overlap between the resonators.
It is stressed that the numbering of elements within the Figures attempts to use same numbers for similar functional elements across the drawings, according to the following list:                100—transmission line (TL)        101—grounded end of a transmission line        102—open end of a transmission line        200—signal strip        201—ground layer        202—dielectric layer        203—vertical interconnect between layers        204—ground strip in a CPW layer        205—coplanar waveguide (CPW) layer        300—a multilayer filter (MLF)        310—a layer within a multilayer filter        320—a stepped impedance resonator (SIR) in a MLF        330—inductive section of a SIR        340—capacitive section of a SIR        350—in/out feed lines of the MLF        400—filter response        